The invention concerns a combined oven for cooking foods suitable for either microwave cooking, cooking by heating the chamber by means of combustion of inflammable gases, or cooking by means of a simultaneous combination of these cooking processes to create particular conditions and cooking effects on foods. These processes can coexist and function also in the presence of a process called "steam cooking" that is, with the injection of water vapor into the cooking chamber.
In the course of this description, particular reference will be made to a cooking oven for cooking food for large groups of people, but what is described and claimed here applies to all kinds of ovens for cooking food, as long as they are provided with gas heating and use microwaves. It is known that oven cooking of food takes on very diverse forms according to the results and effects that one desires to obtain. Among the most preferred methods are microwave cooking and heating by means of combustion, external to the cooking chamber, of inflammable gases and subsequent heat exchange with the interior of the chamber. The peculiarities of these diverse methods of cooking, from the view points of speed, cleaning, safety, energy consumption, and uniformity, are well known and will not be mentioned here.
To summarize the advantages of these two methods, various types of combined gas-microwave ovens have been disclosed, but they have some inconveniences in construction and use, as will be explained in the following.
Combined microwave and gas-combustion ovens are known from patents U.S. Pat. Nos. 4,431,889 and 4,430,541, in which a gas burner positioned outside the cooking chamber generates hot combustion products that are aspirated from the combustion zone by a ventilator. The ventilator pushes these products into the cooking chamber through suitable openings in the chamber walls. Both the exit and entrance openings of the cooking chamber have dimensions substantially less than half the wavelength of the microwaves. Circulation of heated gases inside the cooking chamber is such as to assure substantial uniformity in the circulation of the heated gases.
A combined gas-microwave oven is also known from U.S. Pat. No. 4,211,909, in which separate cooking by gas-combustion heating or by microwave is possible, in order to emphasize particular aspects of cooking performance. In an oven of this kind, the gas burner is placed outside the cooking chamber and the combustion products are made to circulate by a ventilator inside the cooking chamber.
Another combined gas and microwave oven is known from European patent (application) no. 0,349,213, in which it is possible to accelerate the time of heating of the oven by means of simultaneous activation of the two different cooking methods. This oven has a gas burner placed outside the cooking chamber and a ventilator that makes the hot combustion gases circulate forcefully within the chamber. From the functional point of view, there is not much difference from the ovens described previously.
A different combined gas and microwave oven is known from patent GB 2179529 (appl.), in which one or more burners are placed outside the cooking chamber. The passage of gas into the chamber occurs without the aid of ventilators and by means of perforated box elements. The box elements prevent the escape of microwaves from the chamber in one direction. In the other direction, the box elements prevent dirtying of the burner with residues emitted by the foods.
All these types of ovens are effective when limited only to combined microwave cooking and cooking by combustion heating of gases. However, present needs, felt particularly in cooking ovens for food for groups, suggest that it would be desirable to perform the methods of gas and microwave cooking in the same cooking chamber simultaneously with a so-called "steam-cooking" method. Steam cooking is well known to experts in the field and consists of introducing a flow of saturated steam into the cooking chamber at a high temperature. In order to be efficient, this method requires that the steam fill the cooking chamber and envelop the food in an atmosphere of saturated steam at a high temperature. This type of cooking is particularly in demand because of such characteristics as speed and economy. However, the presence of saturated water vapor requires that the chamber be essentially hermetically sealed, even if not under pressure. This fact is totally incompatible with the admission of oxygen and combustion gases from outside the chamber. The combustion products should also be emitted to the outside environment. For this reason, none of the ovens described above is compatible with "steam" operation.